重入锁，建议应用的同步方式。相对效率比synchronized高。量级较轻。
synchronized在JDK1.5版本开始，尝试优化。到JDK1.7版本后，优化效率已经非常好了。在绝对效率上，不比reentrantLock差多少。
使用重入锁，必须必须必须手工释放锁标记。一般都是在finally代码块中定义释放锁标记的unlock方法。
锁的实现：
Synchronized是依赖于JVM实现的，而ReenTrantLock是JDK实现的，有什么区别，说白了就类似于操作系统来控制实现和用户自己敲代码实现的区别。前者的实现是比较难见到的，后者有直接的源码可供阅读。
性能的区别：
在Synchronized优化以前，synchronized的性能是比ReenTrantLock差很多的，但是自从Synchronized引入了偏向锁，轻量级锁（自旋锁）后，两者的性能就差不多了，在两种方法都可用的情况下，官方甚至建议使用synchronized，其实synchronized的优化我感觉就借鉴了ReenTrantLock中的CAS技术。都是试图在用户态就把加锁问题解决，避免进入内核态的线程阻塞。
功能区别：
便利性：很明显Synchronized的使用比较方便简洁，并且由编译器去保证锁的加锁和释放，而ReenTrantLock需要手工声明来加锁和释放锁，为了避免忘记手工释放锁造成死锁，所以最好在finally中声明释放锁。
锁的细粒度和灵活度：很明显ReenTrantLock优于Synchronized
例
/**
* ReentrantLock
*/
package com.sxt.concurrent.t03;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;
public class Test_01 {
Lock lock = new ReentrantLock();
void m1(){
try{
lock.lock();
for(int i = 0; i < 10; i++){
TimeUnit.SECONDS.sleep(1);
System.out.println("m1() method " + i);
}
}catch(InterruptedException e){
e.printStackTrace();
}finally{
lock.unlock();
}
}
void m2(){
lock.lock();
System.out.println("m2() method");
lock.unlock();
}
public static void main(String[] args) {
final Test_01 t = new Test_01();
new Thread(new Runnable() {
@Override
public void run() {
t.m1();
}
}).start();
try {
TimeUnit.SECONDS.sleep(1);
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
new Thread(new Runnable() {
@Override
public void run() {
t.m2();
}
}).start();
}
}
/**
* 尝试锁
*/
package com.sxt.concurrent.t03;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;
public class Test_02 {
Lock lock = new ReentrantLock();
void m1(){
try{
lock.lock();
for(int i = 0; i < 10; i++){
TimeUnit.SECONDS.sleep(1);
System.out.println("m1() method " + i);
}
}catch(InterruptedException e){
e.printStackTrace();
}finally{
lock.unlock();
}
}
void m2(){
boolean isLocked = false;
try{
isLocked = lock.tryLock(5, TimeUnit.SECONDS);
if(isLocked){
System.out.println("m2() method synchronized");
}else{
System.out.println("m2() method unsynchronized");
}
}catch(InterruptedException e){
e.printStackTrace();
}finally{
if(isLocked){
lock.unlock();
}
}
}
public static void main(String[] args) {
final Test_02 t = new Test_02();
new Thread(new Runnable() {
@Override
public void run() {
t.m1();
}
}).start();
try {
TimeUnit.SECONDS.sleep(1);
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
new Thread(new Runnable() {
@Override
public void run() {
t.m2();
}
}).start();
}
}
/**
* 可打断
*/
package com.sxt.concurrent.t03;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;
public class Test_03 {
Lock lock = new ReentrantLock();
void m1(){
try{
lock.lock();
for(int i = 0; i < 5; i++){
TimeUnit.SECONDS.sleep(1);
System.out.println("m1() method " + i);
}
}catch(InterruptedException e){
e.printStackTrace();
}finally{
lock.unlock();
}
}
void m2(){
try{
lock.lockInterruptibly();
System.out.println("m2() method");
}catch(InterruptedException e){
System.out.println("m2() method interrupted");
}finally{
try{
lock.unlock();
}catch(Exception e){
}
}
}
public static void main(String[] args) {
final Test_03 t = new Test_03();
new Thread(new Runnable() {
@Override
public void run() {
t.m1();
}
}).start();
try {
TimeUnit.SECONDS.sleep(1);
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
Thread t2 = new Thread(new Runnable() {
@Override
public void run() {
t.m2();
}
});
t2.start();
try {
TimeUnit.SECONDS.sleep(1);
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
t2.interrupt();
}
}
公平锁
公平锁
/**
* 公平锁
*/
package com.sxt.concurrent.t03;
import java.util.concurrent.locks.ReentrantLock;
public class Test_04 {
public static void main(String[] args) {
TestReentrantlock t = new TestReentrantlock();
//TestSync t = new TestSync();
Thread t1 = new Thread(t);
Thread t2 = new Thread(t);
t1.start();
t2.start();
}
}
class TestReentrantlock extends Thread{
private static ReentrantLock lock = new ReentrantLock(true);
public void run(){
for(int i = 0; i < 5; i++){
lock.lock();
try{
System.out.println(Thread.currentThread().getName() + " get lock");
}finally{
lock.unlock();
}
}
}
}
class TestSync extends Thread{
public void run(){
for(int i = 0; i < 5; i++){
synchronized (this) {
System.out.println(Thread.currentThread().getName() + " get lock in TestSync");
}
}
}
}
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